A major aspect of the maintenance of blood sugar levels is gluconeogenesis. We are studying the pathways by which organic and amino acids and other compounds are converted to glucose in the liver and kidney. Several unresolved aspects are under study: the rate and controls of the transfer of intermediates between the mitochondria and the cytosol; the shuttles by which reducing hydrogen is transferred into and out of the mitochondria; and the method and extent to which glycolysis is controlled in these tissues while net gluconeogenesis is occurring. The basic method involves fitting of isotopic data to complex models of intermediary metabolism with the aid of a digital computer. Specifically labelled substrates such as lactate-2-C14 are incubated with isolated liver parenchymal cells or with the isolated organ and patterns of specific activity are determined in the products and intermediates formed, such as glucose, alanine, glutamate, malate and aspartate. Also use of tritiated substrates which form NADT are employed, with the distribution in glucose being determined by whether the compound is initially metabolized in the mitochondria or the cytosol.